STUDY GUIDE
Atoms, Elements, Molecules, and Bonds
Atoms:
In simple terms, an atom is the smallest chemical building block with a
unique identity. That is to say, atoms are different from each other - an atom of
carbon is different from an atom of nitrogen. It's made of positively charged
protons, neutral neutrons, and negatively charged electrons. Unlike atoms, every
proton is alike. Likewise, every electron is alike. Opposite charges attract each
other and like charges repel (push away), so protons and electrons are attracted
to each other. This attraction is critical for chemistry!
Practice Question 1:
In the center of an atom, you will often find lots of protons. How would
you expect two protons to interact with each other, based on the information
given above? (This class doesn't cover the interactions that actually hold together
the center of the atom!)
Elements:
The number of protons that are in the middle of an atom give it special
properties. For this reason, we say that atoms with the same number of protons
are all of the same element. The number of protons in an atom of an element is
called its atomic number, and is shown above its abbreviation (chemical
symbol) on the periodic table of elements.
Practice Question 2:
Find a periodic table of elements on the Internet and answer the following
questions:
a)
What is the chemical symbol of phosphorus?
b)
Which element has the chemical symbol Na?
c)
How many protons are in an atom of oxygen?
d)
How many protons are in an atom of C?
STUDY GUIDE
Atoms, Elements, Molecules, and Bonds
Counting Electrons:
A neutral atom does not have extra positive or negative charge. That is to
say, it has the same number of protons and electrons. Atoms and molecules that
are charged (not neutral) are called ions.
We can count the total number of electrons in a neutral atom by counting
its protons. The number of electrons is equal to that.
Practice Question 3:
How many total electrons are there in a neutral atom of oxygen?
Ions:
Electrons are negatively charged. Therefore, if an atom gains an extra
electron, it will become negatively charged. If an atom loses an electron, it will
become positively charged (because it lost a negative charge, and now has more
protons than electrons.) It is possible to gain or lose more than one electron!
Practice Question 4:
How many electrons does hydrogen have? How many electrons does a
positive hydrogen ion (H+) have, if any?
Valence Electrons:
However, what matters most for how atoms interact with each other is not
their total number of electrons. We model the electrons of atoms as being
organized into shells, which we imagine to be nested around the center of the
atom like the layers of an onion. What matters most is the number of electrons in
the outermost layer/shell, the valence electrons.
When reading the periodic table, we count rows to find the number of
shells. To find the number of valence electrons in the elements we care about for
this class, we count across columns, skipping the transition metals in the middle
of the table (columns 3-12). That is to say, we only count the first two and last six
columns. For example, nitrogen (N) is in the second row, so it has two electron
shells. It's in the fifth column (if we skip those metals), so it has five valence
electrons (outer electrons).
Most shells hold eight electrons. However, the first one (represented by
the first row of the table) holds only two electrons. Putting that together, nitrogen
has seven protons and seven total electrons. Two of those electrons are in its
inner shell and five are in its outer shell (5 valence electrons). Here is a sketch:
STUDY GUIDE
Atoms, Elements, Molecules, and Bonds
Practice Question 5:
Using a periodic table to guide you, draw an electron shell diagram for
sulfur (S):
Predicting How Electrons Will Move:
For the elements we work with in this class, atoms are most stable when
they have eight valence electrons. If an atom has almost eight valence electrons,
it will strongly tend to attract more electrons to itself to fill its outer shell. For
example, if it has seven valence electrons, it only needs one more electron to have
a full set! So it will tend to attract electrons very strongly. Oxygen, which has 6
valence electrons and therefore only needs 2 more for a full shell, tends to do
this!
However, if an atom has very few (less than 4) valence electrons, it will
instead tend to lose those electrons. Once it has lost all its original valence
electrons, its original outer shell is gone. This means that what used to be an
inner shell is now the new outer shell - and that outer shell is full! The diagram
showing the formation of sodium chloride in the text (chapter 2) illustrates this.
Practice Question 6:
a) How many valence electrons does Fluorine (F) have?
b) Would it tend to gain or lose electrons in order to become stable?
c) How many electrons would fluorine usually gain or lose?
d) Following the diagram for the interaction of Na and Cl shown in chapter
2 of the text, illustrate what happens when potassium (K) and fluorine (F)
interact. Show their shells before and after the gain/loss of electrons:
STUDY GUIDE
Atoms, Elements, Molecules, and Bonds
Molecules:
Molecules are combinations of atoms that have been stuck together. We
write a molecule's formula by listing the chemical symbols of the atoms involved,
listed by element, as well as the count of each kind of atom as a subscript (small
number to the right of the symbol). If no subscript is given, you can assume that
there is only 1 atom of that element. For example, H2O = 2 hydrogen atoms and 1
oxygen atom.
Practice Question 7:
Which elements are present in glucose, C6H12O6? How many atoms of each
element are present?
Covalent Bonds:
When electrons are shared, a force is created that holds molecules
together. Since electrons are attracted to protons, electrons from one atom can be
drawn towards the protons of another, and vice versa. This attraction is called a
covalent bond, and is what holds together, say, the hydrogens and oxygen in a
single molecule of H2O.
Electrons are generally shared in pairs. When a single pair of electrons is
shared, we say that a single bond has been formed. Sharing two pairs of electrons
produces a double bond, and sharing three pairs makes a triple bond. Whether an
atom can make single, double, or triple bonds depends on how many valence
electrons it has and needs! For example, oxygen has 6 valence electrons, and
needs 2 electrons. This means it can form 2 bonds, each bond sharing one
electron with a partner atom (which provides an electron of its own). Oxygen can
thus make 2 single bonds or 1 double bond. See the diagram in chapter 2 of the
text for examples. Carbon is very versatile, because it can make 4 bonds total.
Practice Question 8:
Using the diagrams in the text as a guide, draw the sharing of electrons
between electron shells that occurs in carbon dioxide, CO2. It has the form
O=C=O; both bonds are double bonds. Hint: First draw the inner shells, which
don't participate in the bonding, then draw the valence (outer) electrons. How
many are shared between each C and O?
STUDY GUIDE
Atoms, Elements, Molecules, and Bonds
Hydrogen Bonding:
Not all sharing of electrons is equal! Oxygen tends to pull electrons
strongly to itself. Nitrogen does to a lesser degree; carbon doesn't as much. So
any bond between O and H, or N and H, will have unequal sharing of electrons.
The oxygen (or nitrogen) atom will tend to have extra electrons, on average, and
be negatively charged. The hydrogen will tend to be positively charged.
What this means is that if one molecule has a negatively charged oxygen or
nitrogen, and another molecule has a positively charged hydrogen, those
molecules will tend to attract. This attraction is called a hydrogen bond. It's not
nearly as strong as a covalent bond, but it's still noticeable. This bonding is
responsible for many of the properties of water, such as the difficulty of raising its
temperature, its resistance to evaporation, and the way it dissolves many
hydrophilic (water-loving) molecules easily.
Key point: Covalent bonding occurs within a molecule. Hydrogen bonding
occurs between two molecules. This is illustrated by the diagram in chapter 2 of
the text labeled "hydrogen bonding in water"; you may want to re-examine it
before attempting the next question.
Practice Question 9:
We've drawn the hydrogen bonding of water many times, but what about
the hydrogen bonding of ammonia? Ammonia, NH3, is a base that helps regulate
acidity in the blood. It consists of three hydrogen atoms connected by single
bonds to a nitrogen atom.
a)
Which part of ammonia would be slightly negative in its charge, the
nitrogen or the hydrogen? Which part would be slightly positive?
b)
Draw three ammonia molecules below. Show charges, and illustrate
covalent bonds as solid lines, and hydrogen bonds as dotted lines.